Proton exchange membrane fuel cells (PEMFCs) are used for a variety of mobile and transport applications owing to their substantially high energy conversion efficiency, low emissions and relatively low operating temperatures. In a typical PEMFC, hydrogen gas is supplied to the anode and oxygen gas is supplied to the cathode of the fuel cell. Hydrogen is oxidized to form protons while releasing electrons into an external circuit. Further, oxygen is reduced at the cathode to form reduced oxygen species. Protons travel across a proton-conducting membrane to the cathode to react with reduced oxygen species forming water.
A PEMFC employs a polymer membrane that is ionically conducting and electrically insulating in nature that channels the positive and negative charges during operation of the PEMFC. In order for the oxidation and reduction reactions in the fuel cell to occur at desired rates, electrocatalysts are required. Typically electrocatalysts are coated on the anode and cathode electrodes and a polymer electrolyte membrane is disposed between the anode and the cathode electrodes to form a membrane electrode assembly (MEA). Typically, PEMFCs use noble metals such as platinum as electrocatalysts to facilitate the fuel oxidation and oxidant reaction. Unfortunately, such electrocatalysts are substantially expensive and are not durable thereby inhibiting their use in large-scale applications of fuel cells.
Some PEMFCs use a combination of platinum and a transition metal such as iron, nickel and cobalt to enhance the oxygen reduction reaction (ORR) activity while reducing the overall amount of platinum in the electrocatalyst. However, the stability of such alloy electrocatalysts is substantially poor due to leaching of the transition metals in acid or alkaline media along with poor stability of the catalyst supporting material.
Moreover, current chemical reduction techniques such as sodium borohydride reduction and conventional ethylene glycol reduction used for reducing a metal precursor to nanoparticles take long time for reduction of the precursor. Moreover, these techniques do not provide complete reduction of the precursor resulting in agglomeration of the nanoparticles on the graphene surface. Other electrocatalyst supporting materials used in current PEMFCs include carbon nanotubes and graphitic mesoporous carbon. Again, high costs and low stability of such materials renders them unsuitable for certain applications.
Another electrocatalyst supporting material currently used for fuel cell applications is graphene. Typically, graphene surface is chemically modified to enable deposition of metal nanoparticles on the surface. Such surface modifications are performed using processes such as acid oxidation, ionic liquid linking and plasma treatments. However, most of these surface treatments result in destruction of the graphene structure leading to a decrease in its surface area and reduced electrical conductivity. Moreover, some of these surface treatment processes may often require additional steps before metal nanoparticles deposition on the graphene surface adding to the overall processing costs.